August 7, 2003
09:00 AM (EDT)

News Release Number: STScI-2003-21

Too Close for Comfort

August 7, 2003: This Hubble Space Telescope view of the core of one of the
nearest globular star clusters, called NGC 6397, resembles
a treasure chest of glittering jewels. The cluster is located
8,200 light-years away in the constellation Ara. Here, the stars
are jam-packed together. The stellar density is about a million
times greater than in our Sun's stellar neighborhood. The stars
in NGC 6397 are also in constant motion, like a swarm of angry
bees. The ancient stars are so crowded together that a few of
them inevitably collide with each other once in a while. Near
misses are even more common.

Q & A: Understanding the Discovery

1.
How close are the stars in the center of the globular cluster?

The stellar density is about a million times greater
than in our Sun's stellar neighborhood. The stars are
only a few light-weeks apart, while the nearest star to
our Sun is over four light-years away.

2.
How often do stars collide with each other in NGC 6397?

The ancient stars are s
o crowded together that a few of them
inevitably collide with each other once in a while. Near misses
are even more common. Even so, collisions only occur every few
million years or so. That's thousands of collisions in the
14-billion-year lifetime of the cluster.

3.
What is a "blue straggler"?

When direct collisions occur between two stars, they may
merge to form a new star called a "blue straggler"; these hot,
bright, young stars stand out among the old stars that make up
the vast majority of stars in a globular cluster.

4.
What is a "cataclysmic variable"?

If two stars come close enough together without actually
colliding, they may "capture" each other and become
gravitationally bound. One type of binary that might form
this way is a "cataclysmic variable" a pairing of a normal,
hydrogen-burning star and a burned-out star called a white dwarf.
In a binary system, the white dwarf will pull material
off the surface of the normal star. This material encircles
the wh
ite dwarf in an "accretion disk," and eventually falls
onto it. The result of this accretion process is that cataclysmic
variables are, as the name suggests, variable in brightness. The
heat generated by the accreting material also generates unusual
amounts of ultraviolet and blue light.

5.
How were the Hubble images of NGC 6397 used to search for cataclysmic variables?

To search for cataclysmic variables, the Hubble program consisted of
a series of 55 images of the cluster taken over a period of about
20 hours. Most of the images were taken in ultraviolet and blue
filters; a few images were also taken at green and infrared
wavelengths. By comparing the brightness of all the stars in all
the images, the Hubble astronomers were able to identify several
cataclysmic variable stars in the cluster. Comparison of their
brightness in the different filters confirmed that they were
emitting copious amounts of ultraviolet light. A few of these
stars can be seen in the Hubble Heritag
e image as faint blue
or violet stars.

6.
Where is NGC 6397 located?

The globular cluster is located 8,200 light-years
away in the constellation Ara.